Absorption spectra of actinide compounds

From the energy levels of the gaseous ions Ac⁺⁺, Th⁺⁺⁺, and Th⁺⁺, it can safely be extrapolated that the ground electron configuration of all actinide ions with ionic charges at least + 3 contains only 5f-electrons outside the emanation shells. This is further supported by the 5fⁿ → 5f^n-16d transitions observed in Pa(IV), U(III), Np(III) (though their wave numbers are too high to be measured in U(IV) and Np(IV)). The configuration f² has a very characteristic distribution of energy levels between ³H₄ and ³P₂ with ¹S₀ as much higher, isolated level. The absorption spectra of U(IV) complexes and of PuF₆ suggest interelectronic repulsion parameters ～60 per cent of the values in the corresponding lanthanides, while the Landé factors are about twice as large. This would also explain the recent measurements of Cm(III) complexes, compared to Gd(III). The larger average radius of the 5f-shell causes larger perturbation effects from the ligands than in the lanthanides. The J-values will therefore be distributed between several adjacent, observed energy levels without making the general treatment of the fⁿ-configurations impossible. The variation with oxidation numbers in isoelectronic series are discussed, e.g. U(III) and Np(IV); Pu(III) and AmF₄; and Am(III) and CmF₄. Band positions are predicted in Bk(III) and BkF₄. Finally, the chemical properties of the actinide elements are discussed, and it is emphasized that no necessary connections exist between the presence of f-electrons and constant trivalency.     

Pressure effects on intermetallic actinide compounds

Abstract

Effects of volume compressions on the electronic structure have been studied theoretically for a number of different actinide systems. For the elemental metals the dhcp→fcc crystallographic transformation for Am, Cm, Bk and Cf is calculated and compared with experiment. The magnetism of actinide compounds has also been studied. The spin and orbital 5f moments in UFe₂ are found to be almost equal in magnitude but of opposite directions, which now has been confirmed by neutron experiments. Calculation of the 5f spin polarization in UW, suggests that the 5f localization remains stable even at high pressures. The orbital and spin composition of the magnetic moment in NpOs₂ is studied as a function of volume.     

Magnetic and structural properties of actinide compounds

In this review magnetic and related properties of some actinide metals and solid compounds investigated during 1975–1976 are presented. Compounds with the cubic NaCl, Th₃P₄, CaF₂, Laves phase, AuCu₃ and AuBe₅-type of structure are discussed as well as compounds with lower symmetry. Special attention is devoted to the noncubic uranium compounds in which a singlet-ground-state is supposed to be the origin of their complicated magnetic properties.     

Magnetism and superconductivity in intermetallic uranium compounds

Superconductivity, spin-fluctuation effects and magnetic order are reviewed for intermetallic compounds of uranium with d-transition elements. Large values for the effective mass of the pairing electrons are deduced from critical field studies on the superconducting compounds. Magnetism in the 5f-electron compounds is strongly anisotropic even for paramagnetic materials. Magnetic order does not frequently occur in these intermetallics, spin fluctuations are often observed. A unique combination of spin-fluctuation phenomena and superconductivity is met in UPt₃.     

The relativistic equation of state of actinide metals and compounds

Abstract

The effect of spin-orbit coupling upon the calculated pressure-volume relationship is summarized. It is shown that the main effect is to increase the lattice constant, especially in Pu systems. The both measured and calculated pronounced minimum in the lattice constants of the Anlr₂ laves phase compounds at Nplr₂ is also discussed. One influence of spin-orbit coupling upon the magnetic equation of state is shown to be suppression of magnetism. Again the effects are particularly large in Pu systems. The foregoing is illustrated by a relativistic Stoner theory which explains well the absence of magnetism in Np metal and in several Pu compounds.     

Magnetism,disorder and frustration in heavy-fermion compounds

We examine the magnetic properties of four strongly correlated electron compounds based upon the presence or absence of crystallographic disorder and geometric frustration. All four materials (URu2Si₂, CePd₂Al₃, UNi₄B and URh₂Ge₂) exhibit unusual and not fully understood behavior in their magnetic ordering that are surprising for stoichiometric intermetallic compounds. We summarize these behaviors and relate them to the appearance of disorder and frustration     

Fermi surface,magnetic, and superconducting properties in actinide compounds

The de Haas–van Alphen effect, which is a powerful method to explore Fermi surface properties, has been observed in cerium, uranium, and nowadays even in neptunium and plutonium compounds. Here, we present the results of several studies concerning the Fermi surface properties of the heavy fermion superconductors UPt₃ and NpPd₅Al₂, and of the ferromagnetic pressure-induced superconductor UGe₂, together with those of some related compounds for which fascinating anisotropic superconductivity, magnetism, and heavy fermion behavior has been observed.     

On the temperature dependence of the electrical resistivity of non-stoichiometric actinide compounds

We suggest that the systematic investigation of the dependence of resistivity upon temperature in non-stoichiometric actinide compounds could bring some information on the possibility of non-Kondo minima in highly resistive systems suggested by a recent theory of Markowitz.     

Structural properties of lanthanide and actinide compounds within the plane wave pseudopotential approach

We show that plane wave ultrasoft pseudopotential methods readily extend to the calculation of the structural properties of lanthanide and actinide containing compounds. This is demonstrated through a series of calculations performed on UO, UO2, UO3, U3O8, UC2, alpha-CeC2, CeB6, CeSe, CeO2, NdB6, TmOI, LaBi, LaTiO3, YbO, and elemental Lu.     

Valence fluctuation and heavy fermion behaviour in rare earth and actinide based compounds

A brief historical review of some of the experimental work reported on the mixed valent (MV) or valence fluctuating (VF) and heavy fermion (HF) systems is given. The characteristic physical properties of MV and HF systems are discussed. The salient features of the theoretical models are outlined. Results on some systems are presented.     

Kondo effect,spin dynamics and magnetism in anomalous rare earth and actinide compounds

The problem of magnetic ordering in Kondo lattices is discussed. Criteria of ferro- and antiferromagnetism in the mean-field approximation for thes-f exchange model are obtained. In the case of a constant bare DOS, the saturated Kondo ferromagnetic solution is energetically stable at small intersite interactionsJ. The role of spin-fluctuation corrections at lowT _K in the building up the low-energy scale and the ground state magnetization is considered. Renormalization of spin-fluctuation frequencies atT>T _K is calculated. A possible picture of the formation of magnetism is presented starting from the high-T limit.     

Kondo effect,spin dynamics and magnetism in anomalous rare earth and actinide compounds

The influence of spin dynamics on the Kondo effect manifestations in the Kondo lattices is investigated within perturbation theory with respect to thes-f interaction. It may give rise to Kondo-like divergencies in the electron self-energy already in the second order, resulting in an appreciable effective mass enhancement. As for usual Kondo contributions to thermodynamic and transport properties, the effect of spin dynamics reduces roughly to the replacement ln , with the characteristic spin-fluctuation energy. The thermoelectric power of dense Kondo systems is discussed. Singular contributions to the electron self-energies in the ferro-and antiferromagnetic state are considered. Kondo-like corrections to the intersite exchange interactions, saturation magnetic moment and total energy in a magnetically ordered state are calculated. The strong-coupling regionT<T _K is investigated within the Anderson lattice model. A decrease ofT _K due to spin fluctuations is demonstrated.     

5f Localization in light actinide compounds studied by L3-edge x-ray absorption

L₃-edge x-ray absorption spectroscopy was applied to electronic structure studies in compounds of U, Np, and Pu. Single-peaked white lines are observed at the L₃ thresholds, which exhibit shifts up to 6 eV to lower energies for the monopnictides and monochalcogenides relative to the tetravalent dioxides. These chemical shifts increase systematically from U to Np and Pu, reflecting an increasing localization of the 5f states.     

Volume compressibilities of actinide materials

Abstract

Using an X-ray diffractometer of the Guinier type the bulk moduli B,₀ of NpAs (NaCl structure) and of NpSn₃ (cubic AuCu₃ structure) were measured at room temperature. We find B₀ = (75.3±5.0)GPa, B₀' = (3.7 ± 1.0) for NpAs and B₀ = (72.2 ± 11.6) GPa, B₀' = (5.6 ± 4.1) for NpSn₃. Compared to the results on NpA1₂ and NpOs₂ the B₀ value for NpSn₃ is surprisingly small and supports the interpretation of high pressure Mössbauer data that this intermetallic behaves like a localized 5f electron system. In addition, we report on experimental modifications of the diffractometer which make compressibility measurements possible also at low (down to ～ 10 K) temperatures.     

物质的磁性

1两种假说 a.库仑的"元磁体"假说库仑认为物质是由无数个微小的"元磁体"组成的,每个"元磁体"都有一个N极和S极.物质的磁性是由于元磁体有规则排列的结果.磁性物质内部无数"元磁体"的N极和S极彼此首尾相接,致使物质内部的各元磁体的磁极所显示的磁性彼此抵消,而使磁性物质两端显示极性.非磁性物质是因为组成物质的无数元磁体排列是杂乱无章的,不仅在中央部分它们的磁极作用相互抵消,而且在物体的两端各元磁体的磁极作用也彼此抵消了.由于磁极间的作用与正负电荷间的作用相似,人们认为磁性物质磁极中含有磁荷,N极上有正磁荷,S极上有负磁荷,同号磁荷相互排斥,异号荷核相互吸引.一个正磁荷与一个负磁荷构成一个磁偶极子,从而形成了"磁荷观".     

High pressure studies of actinide intermetallics

Hyperfine Interactions - Magnetic properties of NpOs2,NpAl2. NpCo2Si2, NpAs, and NpSn3 are investigated by high pressure Mössbauer spectroscopy using the 60 keV transition in237Np. A wide...     

Detailed spectroscopy of superdeformed actinide nuclei

Progress in the experimental techniques used to investigate superdeformed fission isomers in the actinides allowed for detailed spectroscopic results of collective properties as well as for the identification of the rotational structure of multiphonon vibrational excitations. A novel approach could be established to determine the depth of the second potential well.